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Structures of the rectum and anal canal seen on a coronal section, anterior view of the female pelvis.
Hello everyone! It's Nicole from Kenhub, and welcome to this tutorial on the rectum and the anal canal. Although we're going to be talking about the associated structures a little bit later in the tutorial, for now, we're just going to focus on this section of the digestive tract – that is the rectum highlighted here while the anal canal is this section in green just below.
As you can see, the rectum and the anal canal are the final structures that make up the digestive tract and they play important roles in the defaecation of faeces. They're quite a few structures to point out for each organ so I'm just going to go through the structure of our tutorial for today and, of course, we're going to begin with the rectum and anal canal's function, structure and location followed by their arterial and venous supply as well as their nervous supply. And, of course, after that, we'll discuss the clinical notes before talking about the associated structures that you can see on this image of the rectum and the anus cut in coronal section.
And, of course, let's begin with the rectum. And the rectum is the last portion of the large intestine and the final stop before faeces are eliminated during defaecation. While we talk about it, let's just pop it out and put it into the center just here. As you can see, the rectum begins at the end of the sigmoid colon at the rectosigmoidal junction and ends at the anorectal junction. The rectum begins at the level of around about S2 to S3 and is around about twelve to sixteen centimeters long with a diameter of around about four centimeters and this diameter dilates distally.
Similar to the colon, the rectum is responsible for absorption and absorbs electrolytes, that is, sodium, potassium and chloride as faeces pass through – and you can see the electrolytes moving out into the rest of the body in this little animation just here. In addition to electrolytes, water is also absorbed through the rectum providing hydration and thickening the stool.
But let's move on now and talk about the structure of the rectum, and one of the key anatomical landmarks in the rectum are the flexures which can be defined as bends or curves along its length. Since the rectum sits within several planes, it has flexures that are found along its vertical length as well as flexures laterally, so let's look firstly at the flexures along its vertical length using this sagittal image of the trunk and the pelvis. And, of course, let's pop it into the center so we can look at it a bit more clearly.
And in this image, you can see that the rectum has two flexures in the sagittal plane and these are called the sacral flexure or the dorsal bend which is given its shape by the concavity of the sacrum and the perineal flexure otherwise known as the ventral bend which forms due to the encirclement of the rectum by the levator ani muscle. And the levator ani muscle is located where the rectum transitions to the anal canal which we referred to earlier as the anorectal junction – and you can see it dotted in blue just here.
The levator ani muscle is formed by the puborectalis muscle whose insertions are on the pubic body slinging around the posterior portion of the rectum and in the image on the left, you can see it slinging around the region of the anorectal junction – you can see how it's inserting into the pubic body.
Let's now have a look at the transverse flexures and let's come back to this coronal image of the flexures. And as you can see, the transverse folds of the rectum highlighted in green create the three lateral flexures and these lateral flexures include the superior flexure which convexes to the right, the intermediate flexure which convexes to the left and the inferior lateral flexure which convexes to the right. And, of course, let's pop out our image. So, again, we can see it more clearly and as we mentioned, the superior flexure over here convexes to the right, the intermediate flexure over here convexes to the left and the inferior lateral flexure convexes to the right so that the rectum terminates in a medial position.
Let's also name these green folds. So over here, we have the superior transverse fold, over here we have the middle transverse fold and down here, we have the inferior transverse fold and this little fold up the top just here, the mark of the rectosigmoidal junction. One last thing before we finish with this image, I just want you to know that the movement of the faeces through the rectum flows in this direction.
Let's now talk briefly about the relationship of the rectum to the peritoneum. So the relationship of the rectum with the peritoneum allows it to be divided into thirds and, of course, we have a superior third which is intraperitoneal, a middle third which lies behind the peritoneum, and an inferior third which is extraperitoneal. And again, we're going to pop out our image so we can look at this a little bit more clearly. So in this image, we're going to draw some lines to divide our rectum and, of course, above the most superior line, we have the superior third of the rectum which is intraperitoneal – and we have this orange shape which represents the peritoneum. And when we say the rectum lies intraperitoneally, we mean that it's covered by the peritoneum on its anterior and its lateral surfaces.
Between the superior line and the inferior line, we have the middle part of the rectum which as we mentioned before is behind the peritoneum – so in the middle third – and therefore only the anterior surface of the rectum is covered by the peritoneum. Finally, the lower third of the rectum lies extraperitoneally under the pelvic diaphragm and is not in contact with the peritoneum at all.
And that's all we have for the function, structure and location of the rectum, so let's move on to the anal canal.
So once the faeces have passed through the rectum, it congregates and builds up enough pressure to move into the anal canal. The anal canal as you can see in the image is the terminal region of the digestive tract and it has an important role in defaecation and maintaining faecal continence. Let's pop it out, of course, for our closer look. And as you can see, it's around about three to four centimeter long and like the inferior third of the rectum, it lies completely extraperitoneally – so there's no peritoneum covering the anal canal at all. And as I mentioned before, it begins at the anorectal junction and continues through to the anus.
Let's now zoom in a little bit on this particular section and we're going to talk about the anal canal's zones. Now, the anal canal can be divided into three zones and these include a columnar zone, an intermediate zone and a cutaneous zone, and, of course, we're going to look at these a little bit more closely. So, in our breakout here, you can see the anal canal divided into three, and the first zone we want to look at is the columnar zone which as you can see has numerous folds of mucous membrane-producing column structures. And these columns are connected to each other at the distal end by transverse folds which are also known as anal valves. And behind these valves lie a cavity where the excretory ducts of the anal glands open into.
Let's move on now to the intermediate zone which is divided from the columnar zone by the dentate or the pectinate line – and you can see that outlined in blue just here. The intermediate zone is about one centimeter in width and is made up of anal mucosa. Finally, let's have a look at the cutaneous zone which is made up of perianal skin and progresses to become the anus.
The final structures that control defaecation are the anal sphincters and there are two anal sphincters – an internal sphincter and an external sphincter – and, of course, let's begin with looking at the internal anal sphincter. As you can see in this image, the internal anal sphincter surrounds the upper two-thirds of the anal canal and is made up of smooth muscle. The internal anal sphincter is permanently contracted by sympathetic tone and only relaxes with parasympathetic influence. So if you remember your parasympathetic and sympathetic tones, we have fight versus flight so the fight instinct – the sympathetic instinct – will help you keep your faeces in whereas the flight instinct will help you let it out which makes sense because if you're fighting, you need to keep it in and if you're running, you can let it out.
Let's move on now to the external anal sphincter which surrounds the anal canal like a clamp surrounding the lower two-thirds of the anal canal. And as you can probably see, there is some overlap between the internal and external sphincters and this is to ensure faecal continence – and you can see that overlap in that gray rectangle I have for you there. The external anal sphincter is in close relationship to the puborectalis muscle which we mentioned a little bit earlier, and both the external anal sphincter and the puborectalis muscle are voluntarily controlled.
So now that we're finished talking about the function, location and structure of the rectum and the anal canal, let's talk about their vascularization. So as you can see in this dorsal view of the rectum, pelvis and pelvic blood vessels, the rectum and the anal canal receive blood from three arteries – the superior rectal artery, the middle rectal artery and the inferior rectal artery. So let's begin by discussing the superior rectal artery.
As you can see, the superior rectal artery provides the greatest amount of vascularization to the rectum, continues from the inferior mesenteric artery which is this artery just here, and terminates in the rectum. The superior rectal artery supplies the majority of the rectum and the region of the anal canal above the dentate line. And in this image, the dentate line is in blue down here so this area above the line is supplied by the artery. The middle rectal artery is a branch of the internal iliac artery which you can see just here and this artery supplies the middle rectum and the entirety of the anal canal. The inferior rectal artery branches from the internal pudendal artery which you can see in blue and it supplies the distal rectum and the anal canal below the dentate line.
So now that we're finished talking about the arterial supply of the rectum and the anal canal, let's talk about the venous supply. So in the rectum and the anal canal, the venous blood is initially drained from the rectum and the anal canal by structures known as the rectal venous plexuses and there are three of these plexuses – the internal venous plexus, the external venous plexus and the perimuscular venous plexus. Let's, of course, begin by discussing the internal venous plexus.
So, the internal venous plexus drains the mucosal, submucosal and perianal tissues and is the cause of internal hemorrhoids. We'll talk a little bit more about internal hemorrhoids a bit later in the tutorial but for now, just remember this little fact. The external venous plexus also drains the mucosal, submucosal and perianal tissues but the external venous plexus is formed by the inferior rectal veins, and it's the cause of external hemorrhoids which we'll talk about a little bit later in the clinical notes. A third venous plexus, the perimuscular venous plexus, is external to the muscular wall of the rectum and can be divided into superior, middle and inferior parts, and these parts of this plexus drain into the respective rectal veins. Let's pop this one out and have a little look at this one a bit more clearly.
So as we mentioned, it has a superior part, a middle part and inferior part, and the superior part is drained by the superior rectal vein, the middle part is drained by the middle rectal vein, and the inferior part is drained by the inferior rectal vein. One thing to note about this is not to confuse this plexus with the external venous plexus as the perimuscular venous plexus is the one that's external to the muscular wall of the rectum whereas the external venous plexus lies within its muscular wall.
And let's move on now to talk about the rectal veins and their drainage. So, the superior rectal vein as we saw just now drains the superior part of the perimuscular rectal venous plexus and then after that sends its venous blood into the portal venous system via the inferior mesenteric vein which is highlighted now for you in blue. The middle rectal vein drains the middle portion of the perimuscular rectal venous plexus in the middle rectum and this then drains into the internal iliac vein. Finally, the inferior rectal vein drains from the inferior portion of the perimuscular rectal venous plexus in the distal rectum and it also receives venous blood from below the dentate line in the anal canal. The inferior rectal vein then drains into the internal pudendal vein.
To finish off looking at the anatomy of this region, let's now look briefly at some of the nerves that supply the rectum and the anal canal. So let's start off by looking at the rectum, and the rectum has sympathetic and parasympathetic innervation as we intuited a little bit earlier in the tutorial and within the sympathetic innervation, the rectum has three different pathways and these include the lumbar splanchnic nerves, the superior hypogastric plexus and the inferior hypogastric plexus. Of course, we have parasympathetic supply and the parasympathetic supply of the rectum is made up of two pathways – the pelvic splanchnic nerves and the inferior hypogastric plexus.
Moving on to the anal canal and you can see that the nerves that innervate the anal canal are divided by the dentate line which to remind you is the distal border of the columnar zone – and you can see it lie outlined in blue just here – and above the dentate line, the anal canal is innervated by the inferior hypogastric plexus whereas below the dentate line, the anal canal is innervated by the inferior anal nerves which are branches from the pudendal nerve.
Okay, so now that you have a solid understanding of the anatomy of this region, let's talk about some different pathologies that can affect this region that are clinically relevant. And the first one we wanted to talk about is associated with the rectum and it's a condition called rectal prolapse which is a fairly frequent pathology of the low GI tract and is also a little bit gruesome. And this injury occurs when part or all of the wall of the rectum slides out of place, sometimes exiting the anus, and it can occur to three different degrees. The first one being partial prolapse which consists of the mucosal lining slipping out of the anus, another kind of prolapse is complete prolapse which is when the entire rectal wall slides out of the anus, and the last kind of prolapse is an internal prolapse where one part of the wall slides over another part of the wall which you can see in our image on the right.
Risk factors for rectal prolapse include cystic fibrosis, having had surgery on the anus, malnutrition, and strained bowel movements. And, luckily, for most people, this disorder actually tends to go away on its own and so the prolapse can often simply be pushed back into place.
Let's now talk about some pathologies of the anal canal, and the most important pathology of this region that you should know about is haemorrhoids.
So, haemorrhoids are vascular cushions that are found within the anal canal of healthy individuals and are actually pretty normal. However, if they become swollen and distended, they are referred to as pathological haemorrhoids or piles and these can be very uncomfortable and can cause pain and bleeding. As we mentioned earlier on in the tutorial, there are a couple of different types of haemorrhoids, and the first one being internal haemorrhoids and the second one being external haemorrhoids. Let's just talk about the internal haemorrhoids to start.
The internal haemorrhoids are caused by a dilation of the internal venous plexus which we alluded to earlier in the tutorial. And so when pressure increases in the rectum, the internal venous plexus dilates to look a little bit like this with the black outline being the inner mucosal wall and the blue lines representing the venous plexus. As for symptoms of internal haemorrhoids, they're usually painless and may only be indicated by bleeding.
External haemorrhoids are caused by dilation or thrombosis of the external venous plexus and that usually results in a haemorrhoid like this one just here, which you can see hanging out of the anus. External haemorrhoids are usually more painful than internal haemorrhoids and a thrombosis or strangulation of the external venous plexus can result in the skin in the region turning purple or blue as well as causing significant pain and running the risk of gangrene which, like in this image, can turn the affected skin black.
Pathological haemorrhoids can be identified through performing a rectal examination, however, in severe cases, they may have to be managed with surgery or surgical approach such as a haemorrhoidectomy or haemorrhoid stapling. And you can see an animation of this haemorrhoid stapling on the right just here.
So now that we're finished talking about the rectum and the anal canal, let's look at the other structures in this image – in our coronal image of the rectum and the anal canal – and starting from the top, let's have a quick look at the sigmoid colon.
The sigmoid colon is the terminal portion of the large intestine and, as you can see, is continuous with the rectum. It's the location where faeces are stored before being expelled through the anal canal and as you can see the sigmoid colon is S-shaped and is around about thirty to forty centimeters in length.
The iliacus muscle which is highlighted in green is a flat muscle that runs along the inner surface of the iliac crest and it joins the psoas major muscle to insert into the femur as the iliopsoas muscle – which is our muscle in this little circle just here – and in this image, you can see it traveling superficial to the external iliac artery and vein as it makes its way from its origin in the pelvis and the sacrum to the femur.
The external iliac artery is the main blood supply to the lower limb as it continues down the thigh as the femoral artery around the level of the inguinal ligament. And in this image, we can see the artery as it runs along the surface of the iliacus muscle highlighted in blue in the direction of the inguinal ligament. Going in the reverse direction, the external iliac vein receives the femoral vein in the inguinal ligament and flows into the common iliac vein.
The obturator internus muscle extends from the obturator membrane and its bony margin to the trochanteric fossa. The obturator internus is a muscle of the gluteal and pelvic region, and in this image, you can see the obturator internus as it passes along the interior surface of the pelvis which is highlighted in blue.
Now, let's take a couple of minutes to talk about the ureter, and the ureter is a paired fibromuscular tube transporting urine from the kidneys to the bladder before being expelled from the body via the urethra. And the ureter is around about twenty-five to thirty centimeters long and in this image, we're looking at the pelvic section as it makes its way down the lateral pelvic wall.
Moving on to the extraperitoneal space, the extraperitoneal space can be defined as any space external to the peritoneum that contains loose areolar tissue, and in this image, you can see the peritoneum highlighted in blue with the extraperitoneal space highlighted in green.
The levator ani muscle is a broad muscular tendon sheet that supports the pelvic floor and helps support the pelvic viscera as well as urine and faecal evacuation and continence. And in this image, you can see primarily the pubococcygeal muscle as it slings around the anorectal junction.
The ischioanal fossa is a wedge-shaped fat-filled fossa inferior to the levator ani muscle and superior to the perineum. The borders of the fossa are as follows. The roof is made up of the levator ani, the floor is made up of deep perineal fascia, the medial wall is formed by the external anal sphincter and the lateral wall is formed by the obturator internus.
The pudendal nerve is a mixed nerve arising from S2 to S4. The nerve supplies the perineum and the perineal floor and in this image, you can see the pudendal nerve running through the pudendal canal in the obturator fascia which is this structure just here. And within this pudendal canal, the pudendal nerve is running with the internal pudendal artery and vein as well as the perineal nerve – a branch of the pudendal nerve which arises in the pudendal canal. And it's in the pudendal canal that the pudendal nerve divides into its terminal branches.
The internal pudendal artery primarily supplies the perineum dividing into many terminal branches that supply the rectum and the sexual organs. As we mentioned on the previous slide, it travels through the pudendal canal with the pudendal nerve and the internal pudendal vein and the perineal nerve. The internal pudendal vein as you might guess drains the perineum and travels with the pudendal nerve and the internal pudendal artery and the perineal nerve in the pudendal canal.
And, of course, last but not least, the perineal nerve as the name suggests innervates the perineum as well as the scrotum in males and the labia majora in females and it is, as I mentioned before, a branch of the pudendal nerve that arises in the pudendal canal. So, clearly, in this image, we're looking at a section after the branching of the pudendal nerve.
So we finally finished our tutorial, let's go over some of what we've talked about today. Of course, in this tutorial, we looked at the structure and location and the function of the rectum looking specifically at the flexures of the rectum and there were two types – vertical which have sacral and perineal flexures and lateral or transverse flexures which included the superior intermediate and inferior lateral flexures. We also talked about its peritoneal relationships and remember there was a superior third, a middle third and an inferior third which were all covered differently by the peritoneum which were covered intraperitoneally up top behind the peritoneum in the middle and extraperitoneally down the bottom.
We also talked about the function, structure and location of the anal canal specifically looking at the zones within the anal canal which included the columnar, the intermediate and the cutaneous zones and we also talked about the anal sphincters of which there are two – the internal anal sphincter and the external anal sphincter. And these anal sphincters, of course, helped with defaecation and continence.
We looked at the vasculature, the arterial supply of the rectum and the anal canal, the first artery of which was the superior rectal artery which supplies the superior part of the rectum, the middle rectal artery which supplies the middle part of the rectum and the entirety of the anal canal, and the inferior rectal artery which supplies the distal rectum and the anal canal below the dentate line.
We also talked about the rectum and anal canal's venous supply which includes venous plexuses such as the internal rectal venous plexus and the external rectal venous plexus which you can't confuse with the perimuscular rectal venous plexus which drains the rectum external to the muscular wall and can be divided into three parts. And we also talked about the rectal veins which included the superior rectal vein which drains the superior part of the rectum, the middle rectal vein which drain the middle portion of the perimuscular venous plexus and the middle rectum, and the inferior rectal vein which drains the inferior portion of the perimuscular rectal venous plexus in the distal rectum.
We also talked about the nerves of the rectum and the anal canal and within the rectum, we had both sympathetic and parasympathetic innervation and the sympathetic innervation consisted of the lumbar splanchnic nerves, the superior hypogastric plexus and the inferior hypogastric plexus while its parasympathetic innervation included the pelvic splanchnic nerves and the inferior hypogastric plexus. The anal canal only had parasympathetic innervation and above the dentate line, this was provided by the inferior hypogastric plexus while below the dentate line, this was provided by the inferior anal nerves.
In our clinical notes, we talked about rectal prolapse of which there are three types – partial prolapse where the mucosal wall falls out of the anus, complete prolapse where the entire rectal wall falls out of the anus, and internal prolapse where part of the internal mucosal wall slides over another part.
We finally then talked about our additional structures in the image and that included the sigmoid colon which is the part of the colon superior to the rectum, the iliacus muscle which runs along the interior surface of the pelvis, the external iliac artery which is one of the main arteries of the lower limb, the external iliac vein which is one of the main veins of the lower limb, the obturator internus which is the major muscle of the hip, the ureter which transmits urine from the kidneys to outside of the body, the extraperitoneal space which is any space that's filled with areolar tissue and is not part of the peritoneum, the levator ani which is a major muscle of the pelvic region, the ischioanal fossa which is a wedge-shaped fat-filled fossa inferior to the levator ani muscle, the pudendal nerve which runs in the pudendal canal, the internal pudendal artery which supplies the perineum, the internal pudendal vein which also drains the perineum, and the perineal nerve which is a branch of the pudendal nerve.
That's all we have time for today. Thanks for watching!
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